No abstract

Hippocampal sclerosis (HS) is a common derangement in many patients with temporal lobe epilepsy. As a result of neuronal cell loss in the hilar region of the hippocampus, it is proposed that mossy fibres sprout and re-innervate new regions of the dentate gyrus. This sprouting may cause recurrent excitation that may lead to the generation of seizures. Here, we determined neuronal density, and synaptophysin and glial fibrillary acidic protein (GFAP) immunoreactivity in hippocampal specimens from patients with pharmaco-resistant temporal lobe epilepsy. Patients were classified into two groups: those with severe and those with no HS. Non-epileptic autopsy tissue served as controls. Mossy fibre sprouting was investigated in these two groups of epilepsy patients using Timm's staining and an immunohistochemical staining of the presynaptic growth-associated protein B-50 (also known as GAP-43, neuromodulin, F1). B-50 immunoreactivity in the different sub-areas of the hippocampus was quantified by image analysis. Our results show the following: (i) in both groups of temporal lobe epilepsy patients, there was a significant loss in cell number in all major hippocampal sub-areas compared with autopsy control tissue; (ii) in HS patients, when compared with non-HS patients, there was a further decline in the number of principal cells in all hippocampal sub-areas analysed, which was associated with an increase in GFAP immunoreactivity; (iii) the decline in cell density was accompanied by a reduced number of synaptic terminals; (iv) in the HS group, there were sprouted mossy fibres in the supragranular layer (SGL) of the dentate gyrus; (v) there was an increase in synaptophysin immunostaining in the SGL indicating that functionally active nerve terminals were formed; and (vi) B-50 immunoreactivity was also increased in the SGL in the HS group compared with the non-HS and control groups. These data showed that all temporal lobe epilepsy hippocampi investigated had severe neuronal cell loss which was most dramatic in the HS group, where it was accompanied by a severe loss of synapses. In the HS group, mossy fibre sprouting into the SGL was found. The increase in B-50 immunoreactivity in the SGL indicated that there was still active sprouting. This sprouting was accompanied by an increased density of synapses, indicating that mossy fibre terminals are not only anatomically present, but probably also functional. Thus, functional glutamatergic mossy fibre terminals are in the right position to synapse on to the dendrites of granule cells and thus may contribute to the onset of seizures.

B-50/GAP-43 is a nervous tissue-specific protein, the expression of which is associated with axon growth and regeneration. Its overexpression in transgenic mice produces spontaneous axonal sprouting and enhances induced remodeling in several neuron populations (Aigner et al., 1995;Holtmaat et al., 1995). We examined the capacity of this protein to increase the regenerative potential of injured adult central axons, by inducing targeted B-50/GAP-43 overexpression in Purkinje cells, which normally show poor regenerative capabilities. Thus, transgenic mice were produced in which B-50/GAP-43 overexpression was driven by the Purkinje cell-specific L7 promoter. Uninjured transgenic Purkinje cells displayed normal morphology, indicating that transgene expression does not modify the normal phenotype of these neurons. By contrast, after axotomy numerous transgenic Purkinje cells exhibited profuse sprouting along the axon and at its severed end. Nevertheless, despite these growth phenomena, which never occurred in wild-type mice, the severed transgenic axons were not able to regenerate, either spontaneously or into embryonic neural or Schwann cell grafts placed into the lesion site. Finally, although only a moderate Purkinje cell loss occurred in wild-type cerebella after axotomy, a considerable number of injured transgenic neurons degenerated, but they could be partially rescued by the different transplants placed into the lesion site. Thus, B-50/GAP-43 overexpression substantially modifies Purkinje cell response to axotomy, by inducing growth processes and decreasing their resistance to injury. However, the presence of this protein is not sufficient to enable these neurons to accomplish a full program of axon regeneration.

No abstract

Affinity-purified anti-B-50 protein antibodies were used to study the previously proposed relationship of the phosphorylation state of B-50 protein and polyphosphoinositide metabolism in synaptic plasma membranes. Antibodies were raised against a membrane extract enriched in the B-50 protein and its adrenocorticotropin-sensitive protein kinase, obtained from rat brain. Anti-B-50 protein immunoglobulins were purified by affinity chromatography on a solid immunosorbent prepared from B-50 protein isolated by an improved procedure. The purified antibodies reacted only with the B-50 and B-60 protein, a proteolysis derivative (of B-50), as assessed by the sodium dodecyl sulfate-gel immunoperoxidase method. These antibodies inhibited specifically the endogenous phosphorylation of B-50 protein in synaptic plasma membranes, without affecting notably the phosphorylation of other membrane proteins. This inhibition was accompanied by changes of the formation of phosphatidylinositol 4,5-diphosphate and phosphatidic acid in synaptic plasma membranes, whereas formation of phosphatidylinositol 4-phosphate was not altered. Inhibition by ACTH 1-24 of the endogenous phosphorylation of B-50 protein in membranes was associated only with an enhancement of the phosphorylation of phosphatidyl-inositol 4-phosphate to phosphatidylinositol 4,5-diphosphate. These data support our hypothesis on the functional interaction of B-50 protein and phosphatidylinositol 4-phosphate kinase in rat brain membranes. The evidence shows that purified anti-B-50 protein antibodies can be used to probe specifically the function of B-50 protein in membranes.

The neuronal phosphoprotein B-50/GAP-43 has been implicated in neuritogenesis during developmental stages of the nervous system and in regenerative processes and neuronal plasticity in the adult. The protein appears to be a member of a family of acidic substrates of protein kinase C (PKC) that bind calmodulin at low calcium concentrations. Two of these substrates, B-50 and neurogranin, share the primary sequence coding for the phospho- and calmodulin-binding sites and might exert similar functions in axonal and dendritic processes, respectively. In the adult brain, B-50 is exclusively located at the presynaptic membrane. During neuritogenesis in cell culture, the protein is translocated to the growth cones, i.e., into the filopodia. In view of many positive correlations between B-50 expression and neurite outgrowth and the specific localization of B-50, a role in growth cone function has been proposed. Its phosphorylation state may regulate the local intracellular free calmodulin and calcium concentrations or vice versa. Both views link the B-50 protein to processes of signal transduction and transmitter release.

The neuron-specific phosphoprotein B-50/GAP43 has been implicated in axonal outgrowth, since high levels of B-50/GAP43 are found in growth cones and during development of the nervous system. In adult brain, the B-50 levels are decreased. B-50 is primarily found in axons and presynaptic terminals. It is phosphorylated by protein kinase C, and this process has been implicated in the modulation of membrane signal transduction. During the outgrowth of the pyramidal tract, high levels of B-50 have been reported, whereas a low amount of B-50 persists into the adult stage. By immunoelectron microscopy, using immunogold labeling on cryosections and pre-embedding peroxidase labeling, we examined the distribution of B-50 in the pyramidal tract at the third cervical segment in developing 2-d-old and adult 90-d-old rats. B-50 immunoreactivity was found in axons and growth cones of the outgrowing tract. In the adult pyramidal tract, both unmyelinated and myelinated axons contained B-50 immunoreactivity. The immunogold label was predominantly located at the plasma membrane. Since the peroxidase reaction product was observed exclusively intracellularly, we conclude that the B-50 immunoreactivity is predominantly located at the cytoplasmic side of the plasma membrane of axons and growth cones. The high immunoreactivity in growth cones and axons of the outgrowing pyramidal tract further supports the hypothesis that B-50 plays a role in neurite outgrowth. The presence of B-50 in the adult pyramidal tract cannot merely be attributed to transport to the synapse. Therefore, it is suggested that B-50 plays, in addition, a local, growth-associated role in the adult tract.

Recent studies have demonstrated that phorbol diesters enhance the release of various neurotransmitters. It is generally accepted that activation of protein kinase C (PKC) is the mechanism by which phorbol diesters act on neurotransmitter release. The action of PKC in neurotransmitter release is very likely mediated by phosphorylation of substrate proteins localized in the presynaptic nerve terminal. An important presynaptic substrate of PKC is B-50. To investigate whether B-50 mediates the actions of PKC in neurotransmitter release, we have studied B-50 phosphorylation in intact rat hippocampal slices under conditions that stimulate or inhibit PKC and neurotransmitter release. The slices were labelled with [32P]orthophosphate. After treatment, the slices were homogenized, B-50 was immunoprecipitated from the slice homogenate, and the incorporation of 32P into B-50 was determined. Chemical depolarization (30 mM K+) and the presence of phorbol diesters, conditions that stimulate neurotransmitter release, separately and in combination, also enhance B-50 phosphorylation. Polymyxin B, an inhibitor of PKC and neurotransmitter release, decreases concentration dependently the depolarization-induced stimulation of B-50 phosphorylation. The effects of depolarization are not detectable at low extracellular Ca2+ concentrations. It is concluded that in rat hippocampal slices B-50 may mediate the action of PKC in neurotransmitter release.